The constitutional amendment -- widely seen as the most momentous of the nation's 153 ballot measures -- will limit marriage to heterosexual couples, the first time such a vote has taken place in a state where gay unions are legal.

Gay-rights activists had a rough election elsewhere as well. Ban-gay-marriage amendments were approved in Arizona and Florida, and Arkansas voters approved a measure banning unmarried couples from serving as adoptive or foster parents. Supporters made clear that gays and lesbians were their main target.

In California, with 95 percent of precincts reporting today, the ban had 5,125,752 votes, or 52 percent, while there were 4,725,313 votes, or 48 percent, opposed.

Similar bans had prevailed in 27 states before Tuesday's elections, but none were in California's situation -- with about 18,000 gay couples married since a state Supreme Court ruling in May. The state attorney general, Jerry Brown, has said those marriages will remain valid, although legal challenges are possible.